To demonstrate the efficacy and safety of a cost-effective and convenient treatment regimen for the delivery of aerosol tobramycin to patients with CF who are chronically colonized with P. aeruginosa, and who have evidence of moderate-severe obstructive airway disease. Specifically, this project will attempt to answer the following question: Is the administration of tobramycin by inhalation twice daily for 28 days efficacious and safe in patients with CF who are chronically colonized with P. aeruginosa?